Apparatus for production of a battery paste

ABSTRACT

A process for making a battery paste includes steps of forming an active material in an aqueous slurry, then dewatering the slurry to form a battery paste. In one described embodiment, a slurry containing one or more basic lead sulfates suitable for use as the active material in lead-acid battery electrodes is formed in a continuously stirred tank reactor (36). The slurry is withdrawn from the reactor (36) and fed to a belt press (67) which reduces the moisture content of the slurry to the desired level. Battery pastes produced according to the invention may be fed continuously to a paster (89) for mass production of positive and negative battery plates from grids (91).

This is a division of U.S. Ser. No. 07/357,007 filed on May 25, 1989 nowU.S. Pat. No. 5,096,611.

TECHNICAL FIELD

This invention relates to a process for producing battery paste for themanufacture of battery electrodes, particularly to a process forpreparing positive and negative pastes for making lead-acid batteryelectrodes. The invention further provides an apparatus for carrying outsuch a process.

BACKGROUND OF THE INVENTION

Battery electrodes (plates) used in lead-acid batteries are made byapplying a paste made of a lead compound to the surface of a batteryplate and electro-chemically forming the paste into an active material.Such pastes typically contain lead, lead oxide(s), basic lead sulfatecompounds, and water.

In general, the paste is made by adding sulfuric acid and water to amixture of lead and lead oxide(s) to form basic lead sulfate compoundsin a mixture with excess unreacted lead oxide and lead. According to oneknown process, this is done by first weighing out a predetermined amountof lead oxide into a weigh hopper and dumping the lead oxide into abatch mixer, such as a mulling mixer. Dry additives such as fiber andexpander are directly added into the mixer. The resulting mixture is drymixed for several minutes so that the fiber and expander are dispersedthroughout the oxide. Water is then added as needed to make a paste ofthe desired consistency. Excessively moist or dry paste render pastingimpossible. The wet mixture is mixed for a short time to wet out thelead oxide. Sulfuric acid is then added as mixing continues until thetemperature peaks at about 65° C. and then drops to the range of 43°-49°C. The acid is added gradually to prevent the paste from overheating.The resulting paste is then cooled by evaporation of water andconduction to the mass of the mixer. Such a lead-acid battery paste isgenerally made in a batch reactor, although continuous processes havebeen suggested.

Many variations of this process have been proposed. In one method, apre-sulfated paste material containing basic lead sulfate, e.g., tri-and tetrabasic lead sulfates (3PbO·PbSO₄ ·H₂ O and 4PbO·PbSO₄), is madein dry form prior to forming the paste. See Malloy U.S. Pat. No.3,194,685, issued Jul. 13, 1965, Johnstone U.S. Pat. No. 2,182,479,issued Dec. 5, 1939, and Weir U.S. Pat. No. 1,572,586, issued Feb. 9,1926. Monobasic lead sulfate has also been used as a presulfated pastematerial. Large crystals of monobasic lead sulfate are formed bysulfurizing an aqueous solution of basic lead acetate with, for example,amido sulfonic acid. The monobasic lead sulfate product is then driedprior to its use in preparing a paste mix. See, for example, Voss et al.U.S. Pat. No. 3,169,890, issued Feb. 16, 1965.

Lead oxide has been reacted with ozone to form improved lead oxidesuseful as active materials in batteries, as described in Parker U.S.Pat. No. 4,388,210, issued Jun. 14, 1983 and Mahato et al., U.S. Pat.No. 4,656,706, issued Apr. 14, 1987. Persulphate treatments have alsobeen used to convert lead oxide to lead dioxide in battery plates. SeeReid U.S. Pat. No. 2,159,226, issued May 23, 1939.

In another process a reactor for continuously producing a sulfated leadoxide includes a continuously operating mixer into which a pulverizedfiller (lead-lead oxide powder) is fed by a pneumatic duct or screwconveyor. Sulfuric acid is sprayed into the dry mixture to form thesulfated reaction product. Water is later added to form the batterypaste. See, e.g., Jache U.S. Pat. No. 3,449,166, issued Jun. 10, 1969.

Biagetti U.S. Pat. No. 3,765,943 emphasizes the advantages of preparinga tetrabasic lead sulfate from orthorhombic lead oxide. The lead oxidestarting material and an expander are mixed with aqueous sulfuric acidso that the reaction is carried out in aqueous suspension. See alsoBiagetti et al., Bell System Technical Journal, September, 1970, No. 49,pp. 1305-1319, wherein the pastes are pre-diluted with water just priorto application to cell grids. Positive plates prepared according to sucha procedure generally exhibit good performance and cycle life. However,positive plates prepared from such presulfated paste mixes are difficultto form; see, for example, Yarnell and Weeks, J. Electrochem. Soc., No.126, p. 7 (1979). Such plates must usually be cured for at least 24hours before being formed.

Prior art batch processes suffer from various disadvantages. The mixingvessel must be kept clean to avoid jamming due to dried paste left overfrom a previous batch. Cleaning is also needed in order to switch from anegative plate batch to a positive plate batch because the negativeplate additives reduce the performance of positive plates. Batch methodsalso generally require a dry mixing step prior to the sulfate-formingreaction step. The present invention addresses these disadvantages.

One aspect of the present invention utilizes a dewatering apparatus in aprocess for making battery paste. Many such devices, such as beltpresses, are known and have been used to dewater compositions such assludges. Davis et al. U.S. Pat. No. 4,697,511, issued Oct. 6, 1987,Emson et al. U.S. Pat. No. 3,974,026, issued Aug. 10, 1976, Davis U.S.Pat. No. 4,475,453, issued Oct. 9, 1984, Dahl U.S. Pat. No. 4,705,602,issued Nov. 10, 1987, Wohlfarter U.S. Pat. No. 3,942,433, issued Mar. 9,1976, Bastgen U.S. Pat. No. 4,019,431, issued Apr. 26, 1977, andHakansson et al. U.S. Pat. No. 4,501,669, issued Feb. 26, 1985 exemplifysuch devices. The present invention advantageously employs a dewateringdevice in an apparatus for making battery paste.

SUMMARY OF THE INVENTION

A process for making a battery paste according to the invention includesthe steps of forming lead compounds in an aqueous slurry, and reducingthe water content of the slurry to form a battery paste. According toone aspect of the invention, a slurry containing one or more basic leadsulfates suitable for use as the active material in lead-acid batteryelectrodes is continuously formed in a reactor. The slurry is withdrawnfrom the reactor and fed to a belt press which reduces the moisturecontent of the slurry to the desired level. The invention furtherprovides an apparatus for carrying out such a process, including areactor and a device associated with the reactor for reducing themoisture content of the slurry. The invention additionally provides aslurry composition which can be made by the described process, whichslurry is useful in the manufacture of battery paste.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be further described with reference to theaccompanying drawing, wherein like numerals denote like elements, and:

FIG. 1 is a schematic diagram of an apparatus according to theinvention;

FIG. 2 is a schematic diagram of a control system for the apparatusshown in FIG. 1;

FIG. 3 is a partial schematic diagram of an alternative embodiment of anapparatus according to the invention employing multiple reactors; and

FIGS. 4A and 4B are partial schematic diagrams of further alternativeembodiments of an apparatus according to the invention employingmultiple reactors; and

FIG. 5 is a flow diagram of a process for making a battery pasteaccording to the invention.

DETAILED DESCRIPTION

The present invention provides a process and apparatus for making abattery paste wherein the active material can be continuously mixed inaqueous slurry form. Referring to FIGS. 1 and 2, a process line (system)10 according to the invention for making battery paste includes as majorcomponents a system 5 for making powdered lead oxide (leady litharge)from lead ingots, a lead oxide reactor feeder 6, a reactor 36, adewatering apparatus 67, and an optional control system 100. The natureand function of each of these components is described in detailhereafter.

Referring to FIG. 1, system 5 includes a conventional ball mill 11 whichpulverizes leaf ingots 12 in the presence of air to form lead oxides. Inthe alternative, a Barton pot may be used to make lead oxide particlesby atomiziation of liquid lead. Ball mill 11 continuously feeds the leadoxide powder through a line 13 to a classifier 14. Particles outside thedesired particle size range are fed through a return line 16 back tomill 11. Particles suitable for use in making lead-acid battery pastegenerally have sizes of 150 microns or less, preferably 70 microns orless, most preferably 1-50 microns.

Particles from classifier 14 are fed through a line 17 to a bulk storagetank 18. Tank 18 has a large enough capacity to accumulate lead oxidepowder therein when the apparatus downstream from tank 18 is idle, i.e.,requires no additional oxide. Oxide powder is fed from the bottom oftank 18 by any suitable means, such as a screw conveyor 19, into a weighhopper 22. Conveyor 19 forces the powder through a screen 21 to break uplumps which form during storage in tank 18.

Weigh hopper 22 includes a metering valve 26 which meters the oxidepowder into an oxide feeding tank 23. The level in tank 23 is maintainedby a loss-in-weight feeder 24 which weighs the amount of powder in tank23 and actuates metering valve 26 when the measured weight decreasesbelow a predetermined level. When valve 26 opens, substantially theentire contents of hopper 22 fall into the tank 23, ensuring that theamount of lead oxide in tank 23 is within a desired predetermined range.Hopper 22, tank 23 and loss-in-weight feeder 24 thus cooperate toprovide a uniform supply of lead oxide powder to reactor 36.

The oxide powder from feed tank 23 is fed by a screw conveyor 33 to areactor 36 in which the active basic lead sulfate is formed. The oxideis preferrably fed continuously at a rate in the range of typicallyabout 900 to 4,500 kg per hour. Water is fed into reactor 36 through awater line 37 in an amount sufficient to maintain a slurry at all times,i.e., an amount largely in excess of the amount needed to form the finalpaste product. A level sensor 38 located near the top of reactor 36,such as a mercury float switch or an ultrasonic level detector, actuatesa valve 41 in water line 37. Valve 41 is opened to admit water intoreactor 36 in response to a decrease in slurry level in, reactor 36 asdetected by sensor 38, then closed when the predetermined level isreached.

Several other inlet lines feed additional materials into reactor 36 asneeded to form the desired paste product. Slurries of expander and fiberadditives are prepared in respective tanks 42, 43. Tanks 42, 43 areprovided with respective stirrers 44, 46 which operate as needed, e.g.,continuously, to provide a uniform slurry in each tank. Fibers are usedin both positive and negative paste mixes as a binder to improve thehandling characteristics of the battery plates after pasting. Suitablefibers include fiberglass, tin or tin dioxide-coated fiberglass, carbonfibers, synthetic plastic fibers such as modacrylic fibers, and mixturesthereof. Such fibers preferrably have a fineness of about 3 denier andlengths in the range of 1/8 to 1/16 inch. Specific gravity of preferredmodacrylic fibers according to the invention is in the range of about1.2 to 1.5 gm/cc. Fiber and expander may also be added directly toreactor 36 in dry form.

The expander is added to negative paste mixes to extend the batterycycle life of negative plates. The expander functions by minimizingdetrimental shrinkage and surface area losses which normally occur innegative plate materials during repeated battery charge and dischargecycles. Suitable expanders include carbon black (also a colorant),lignins, or their synthetic equivalents, barium sulfate, and mixturesthereof.

Water and expander or fiber are added to tanks 42, 43 manually (orautomatically) as needed. The ratio of solid to water should be lessthan 60 grams per liter for the fibers, preferably about 24 to 36 gramsper liter to maintain an even, relatively thick slurry. The solid towater ratio for the expander may range from about 0.12 to 1.2 kilogramsper liter, preferably about 0.24 to 0.48 kilograms per liter.

In the embodiment of FIG. 1, reactor 36 is a continuously stirred tankreactor (CSTR) including a reactor vessel (tank) 35 which has adischarge outlet 62A at the side thereof. A plug flow reactor could alsobe utilized. Outlet 62A is preferably located on the side of reactor 36,about halfway down, because free lead particles in the slurry tend tosink to the bottom of the reactor and would clog an outlet located atthe bottom of the reactor. If the lead oxide fed into reactor 36 isessentially lead free, or if the slurry does not contain substantialamounts of tetrabasic lead sulfate, then outlet 62A may be located atthe bottom of reactor 36. Otherwise, a drain valve 62B is provided atthe bottom of reactor 36 for periodically drawing off the heavy,lead-rich slurry that accumulates at the bottom of reactor 36. Thelead-rich slurry drawn from valve 62B may be returned to a smelter forreclamation or reprocessed through an oxide mill.

The slurry process of the invention can remove free lead from theslurry. This lead removal process provides a means for eliminating alengthy curing step which is otherwise needed to lower free lead levelsby air oxidation. The alternative of using a lead-free lead oxidestarting material is generally expensive. The present inventioneliminates the need for such measures.

If desired, pressurized ozone gas from an ozone supply line 32 may befed into reactor 36 near the bottom thereof so that ozone gas bubblesupwardly through the slurry. The ozone reacts with the oxide to convertPbO to PbO₂. This PbO₂ acts as a conductive additive which enhances theformation efficiency of a positive plate. Gaseous ozone may be producedby any suitable means. For example, a gaseous mixture for use in theinvention containing about 6 percent ozone can be readily formed bysubjecting oxygen gas to corona discharge. The resulting gas ispreferably humidified to 100% room humidity for best results. Ozone gascan also be formed electrochemically.

As an alternative, a conductive additive may be added directly to theslurry. Useful conductive additives include PbO₂, Pb₃ O₄, carbon, metaloxides including tin dioxide and titanium oxide, and combinationsthereof. Such a conductive additive is generally added in amountsranging from about 0.1 to 50 percent by weight based on the solidspresent in the paste. The conductive additive may be added manually orby a dry feeder 45 similar, for example, to loss-in-weight feeder 6.

Reactor 36 may be operated at any temperature in the range of 0° C. to100° C. or higher, if reactor 36 is pressurized. Temperatures in therange of about 20° C. to 95° C. are preferred for the synthesis oftri-and tetrabasic lead sulfates. The solids concentration of the slurrygenerally may range from about 10 wt. % to about 80 wt. %. Outside ofthis range the slurry is either excessively thin or thick. The totalsolids added to reactor 36 to form the slurry generally comprise atleast about 98 wt. % lead oxide and less than about 2 wt. % fiber andexpander, if used. The lead oxide component can contain about 0 to 35wt. % free lead as well as lead oxides such as PbO, PbO₂ and Pb₃ O₄. Thefollowing table sets forth preferred reactant compositions for positiveand negative paste mixes:

                  TABLE 1                                                         ______________________________________                                        Solids for Positive Paste                                                     Ingredient   Preferred Range                                                                             Most Pref. Range                                   ______________________________________                                        Fiber        0 to 0.4 wt. %                                                                              0.05 to 0.25 wt. %                                 Cond. Additive                                                                             0 to 50 wt. % 0.1 to 25 wt. %                                    Lead oxide*  balance       balance                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Solids for Negative Paste                                                     Ingredient  Preferred Range                                                                             Most Pref. Range                                    ______________________________________                                        Fiber       0 to 0.4 wt. %                                                                              0.10 to 0.12 wt. %                                  Expander    0 to 3 wt. %  0.50 to 2.10 wt. %                                  Carbon      0 to 0.5 wt. %                                                                              0 to 0.2 wt. %                                      Lead oxide* balance       balance                                             ______________________________________                                         *Includes free lead also.                                                

These solids, in combination with the acid and water present in theamounts described above, provide a slurry according to the inventionuseful in the synthesis of lead-acid battery pastes.

A pair of metering pumps 47, 48 control the flow of expander and fiberthrough respective lines 49, 51. Pumps 47, 48 and other similar pumpsused in system 10, may, for example, be positive displacement pumps,particularly progressing cavity pumps. Each such pump can convert anelectrical signal received from a control device to a rotating speedwhich corresponds to the flow rate therethrough, or to the strokefrequency, if a reciprocating pump is used. Lines 49, 51 are connectedto tanks 42, 43 near the bottom of each tank. Pumps 47, 48 generallyoperate simultaneously with conveyor 33 and feeder 45 (if present) at apredetermined rate which is calculated from the specific paste recipe.Similarly, sulfuric acid is fed directly into reactor 36 at apredetermined rate through an acid feed line 52.

A metering pump 53 controls the rate at which the acid is introducedinto reactor 36. Pump 53 also operates in conjunction with screwconveyor 33 so that the oxide, fiber, expander, conductive additive andacid are added together, preferably to the upper surface of the slurryin reactor 36. No step of premixing the oxide with the fiber andexpander is needed. Recycled scrap paste material from a paster 89 mayalso be added directly into reactor 36.

Reactor 36 has a stirrer 54 which runs continuously to keep the solidsin suspension and prevent the slurry from solidifying on the inside ofvessel 35. A temperature control jacket 56 surrounds the outside ofreactor 36. A heat transfer fluid such as water is continuouslycirculated by a pump 55 from a reservoir 58, through a heat transferfluid supply line 57, jacket 56, and a return line 59 back to reservoir58. Reservoir (water heater tank) 58 is heated intermittently by aheater 60. A thermocouple 61 is provided to determine the temperature ofthe slurry and intermittently actuate heater 60 to maintain the desiredreactor slurry temperature. If a low reaction temperature is desired,heater 60 may be replaced by a suitable cooling unit. As an alternativeto a jacket-type temperature control system, heating elements may beembedded in the wall of reactor 36 if no cooling will be needed.

The slurry is intermittently or continuously withdrawn from outlet 62A.A valve 63 regulates flow from outlet 62A. Preferably, the slurry iswithdrawn at the same rate at which the solid ingredients are added, sothat the composition within the reactor remains constant and thereaction(s):

H₂ SO₄ +5PbO→4PbO.PbSO₄ +H₂ O

H₂ SO₄ +4PbO→3PbO.PbSO₄.H₂ O

H₂ SO₄ +2PbO→PbO.PbSO4+H₂ O

H₂ SO₄ +PbO→PbSO₄ +H₂ O

do not result in complete conversion of all of the lead oxide (PbO) tolead sulfates, as in prior processes. As a result, the slurry withdrawnfrom reactor 36 may contain substantial amounts of both lead oxide andlead sulfates. It is normally preferable to have both of thesecomponents in the resulting battery paste. The illustrated embodiment ofthe invention can thus eliminate the need to convert all of the leadoxide to basic lead sulfate, and the subsequent step of adding leadoxide back in to make the paste.

The slurry then flows through an optional heat exchanger 64, forexample, a shell-and-tube type heat exchanger. Heat exchanger 64 isconnected to a heat transfer medium circulation system (not shown) suchas, for example, one similar to that described above including tank 58,pump 55, and lines 57, 59. Heat exchanger 64 decreases the temperatureof the slurry to the desired paste temperature, generally to about 43°C. to 49° C., the temperature at which a paster usually operates.However, pasting temperatures outside of this operating range may beappropriate for some slurry prepared pastes. The slurry next flowsthrough a pipeline 66 towards a moisture reduction device. In thedescribed embodiment, the moisture reduction device is a vacuum-assistedfilter press 67, such as an expressor press manufactured by EimcoProcess Equipment Company.

Prior to reaching filter press 67, the slurry may be commingled inpipeline 66 with a persulfate additive fed into pipeline 66 through abranch line 68 from a persulfate reservoir 69. The persulfate reactswith lead oxide in the paste to form lead dioxide as follows:

2PbO+Na₂ S₂ O₈ →PbO₂ +PbSO₄ +Na₂ SO₄

An analogous reaction can be written for the potassium persulfate salt.Persulfate addition is a practical alternative to direct lead dioxideaddition. A metering pump 71 disposed in branch line 68 regulates theamount of persulfate added. The persulfate may be in powder form or maybe an aqueous solution of a salt of peroxydisulfuric acid, H₂ S₂ O₈. Ifnecessary, a suitable mixer (not shown) may be provided in line 66 tomix the persulfate additive with the slurry from the reactor. Powderedpersulfate may alternatively be added directly to reactor 36 from dryfeeder 45. The amount of sodium or potassium persulfate is preferablyabout 1 to 10 percent by weight of the solids in the slurry.

Positive plates containing tetrabasic lead sulfate exhibit very gooddischarge capacity and cycle life when they are properly formed.However, these plates are normally very difficult to form. Theseformation problems are especially difficult when the tetrabasic leadsulfate containing plates are made using low free lead oxide.Incorporation of persulfate or one of the other equivalent conductivityenhancers described above into the slurry preparation of such platesyields efficient high rate formations resulting in high performancepositive plates.

It has been determined that free lead levels greater than about 5 wt. %can strongly inhibit the persulfate reaction. Therefore, use of apersulfate additive is generally restricted to low free lead contentoxides. If leady oxides containing higher free lead contents are used,then the previously described ozone treatment or direct addition of aconductivity enhancing agent can be used instead.

Persulfate treatment may be used either alone or in conjunction with theozone treatment or direct addition of a conductivity enhancing agent.All such treatments are better suited for making positive plates ascompared to negative plates. The strong reducing environment of thenegative plate adversely affects plate formation when PbO₂ is formedtherein by the foregoing persulfate or ozone reactions. Normally alengthy curing step can be omitted for negative plates whether or notthey contain large amounts of free lead. Thus, negative plates accordingto the invention do not generally contain lead dioxide.

Negative plate formation poses problems if the lead oxide used to makethe paste has a very low free lead content, for example, less than about0.2 wt. % of the lead oxide. This problem can be remedied byincorporating therein an amount of high surface area carbon effective toenhance the formation process without adversely affecting batteryperformance. For this purpose, from about 0.05 to 0.5 wt. %, especially0.1 to 0.2 wt. % of carbon based on the total solids present may beincorporated into the negative paste mixture. This can be done, forexample, by commingling the slurry in pipe 66 with the carbon additiveby means similar to those described for the persulfate additive, or byadding carbon directly to reactor 36 via feeder 45.

A preferred negative paste mixture contains, as solids, 55-60 wt. %(tri)basic lead sulfate, 40-44 wt. % o-PbO, 0.5-0.8 wt. % expander,0.1-0.2 wt. % of Ketjen black carbon powder having a surface areagreater than about 1200 m² /gm BET, and 0.05-0.2 wt. % plastic fibers.The carbon preferably has a high surface area as described in theforegoing example. The final water content of such a paste is in therange of about 0.1-0.2 ml/gm of solids. The added carbon is in additionto any carbon already present in the expander or fibers.

The slurry is discharged from pipe 66 and uniformly distributed onto theupper surface of a moving conveyor belt 73. Belt 73 is made of amicroporous fabric. A vacuum pump 74 applies suction to the slurry onbelt 73 and draws water therefrom. Suction of at least about 380 mmHg isgenerally needed for this purpose. Excess water is drawn through belt 73into a suitable collection funnel 75 which is in turn connected to arecirculation line 76. A pump 77 operates continuously to return waterfrom collector 75 to reactor 36.

The dewatered slurry then passes beneath a second (upper) conveyor belt81. Belts 73, 81 press-filter water from the slurry. At least one andpreferably both of belts 73, 81 are made of a microporous fabric, suchas nylon, polypropylene or polyester, preferably having an average poresize of about 10 microns or less, preferably 1 micron or less. Thesecharacteristics render belts 73, 81 durable, water pervious andsubstantially impermeable to fine, solid particles in the slurry. Aseries of pairs of upper and lower, spaced-apart parallel pinch rollers82, 83 are disposed above and below belts 81, 73, respectively. Eachpair of rollers 82, 83 exerts at least about 50 pounds per linear inch(88 N/cm) on the paste passing therebetween.

Excess moisture in the paste is squeezed out by rollers 82, 83 and drawnthrough belt 73 into the associated collector 75. The filtrationpressure is adjusted to achieve the desired level of moisture for thepaste, generally 5 to 30 wt. %, most preferably about 12 to 15 wt. % foroptimum grid pasting characteristics. The resulting filtration cake hasa thickness of several millimeters and is suitable for use as alead-acid battery paste.

The paste emerges from beneath upper belt 81 and travels on lower belt73 beneath an infared moisture sensor 86. Sensor 86 measures themoisture of the paste and sends a signal of corresponding magnitude to afilter press controller 106 (see FIG. 2). Controller 106 adjusts thepressure(s) exerted by rollers 82, 83 to maintain the desired moisturelevel in the paste. The pressure is increased if the moisture levelbecomes too high, and decreased if it becomes too low.

The paste is then fed directly from one end of filter press 67 by anysuitable means, for example, over a blade 85, into a conventional pastercone feeder 88. In the alternative, since the invention can supply pasteon demand to paster 89, cone feeder 88 may be eliminated. Cone feeder 88feeds the paste to paster 89, which applies the paste to lead grids 91to make battery plates. Alternatively, if desired, the paste may firstpass through a mill in order to break up the filtration cake forpasting, before being fed to cone feeder 88. An advantage of theinvention is that the moisture content of the paste emerging from filterpress 67 can be precisely controlled, so that no subsequent drying stepis needed.

The foregoing apparatus according to the invention can be used toprovide a continuous supply of battery paste for the manufacture ofpositive or negative battery plates. The operative components of thesystem, including the pumps, conveyors, feeders and stirrers, may beoperated manually. However, in a preferred embodiment of the invention,conventional progammable logic controllers (PLC's) or similar devicesare used to control the operative components of the system.

Referring to FIG. 2, a master controller 101 accepts inputs for thedesired reactor temperature (water heater set point), the feed rate forconveyor 33, the feed rates for the other ingredients as indicated bythe paste recipe, i.e., for the fiber, expander, conductive additive(CA) and acid, the desired moisture level in the paste, and the desiredslurry level in reactor 36. Controller 101 sends the set pointtemperature to thermocouple 61. Thermocouple 61 then actuates heater 60as needed to heat the heat transfer fluid in jacket 56, which in turnheats the slurry in reactor 36 to the desired temperature.

Master controller 101 is connected to a slave reactor supply controller103 which operates conveyor 33, pumps 47, 48 and 53, and feeder 45,which feed expander, fiber, acid, and conductivity enhancer,respectively, to reactor 36. Controller 103 receives the slurry recipefrom master controller 101 as flow rates for conveyor 33, pumps 47, 48and 53, and feeder 45. Master controller 101 is also connected to filterpress controller 106 to both control operation of filter press 67 andprovide filter press controller 106 with the selected paste moisturelevel. Master controller 101 is further connected to reactor dischargevalve (or pump) 63 and reactor level sensor 38. Valve 63 opens andcloses in response to a signal from master controller 101. Mastercontroller 101 sends level sensor 38 the selected slurry level settingfor reactor 36.

Controller 101 receives an input signal (call for paste) from paster 89which indicates that paster 89 is in operation. In the alternative,controller 101 can be connected to paster cone feeder 88. Feeder 88 canbe provided with a level sensor which sends a call for paste to mastercontroller 101 in response to depletion of the supply of paste in conefeeder 88.

Upon receiving a call for paste from paster 89 or feeder 88, mastercontroller 101 opens valve 63 and sends a signal to filter presscontroller 106, causing it to operate filter press 67, i.e., turn onconveyors 73, 81, pressure rollers 82, 83 and vacuum 74. The output flowthrough valve 63 (in slurry volume per unit time) and the slurry density(weight of solids per unit volume) are known constants. Using theseconstants, the desired rate of addition of solids (oxide, fiber,expander) to reactor 36 is determined.

Simultaneously with the signal opening valve 63, master controller 101sends a signal to reactor supply controller 103 which causes it toactuate conveyor 33 and pumps 47, 48 and 53 at their respectivepredetermined rates. The rate of acid addition is calculated based onthe stoichiometry of the reaction occurring in the reactor. Mastercontroller 101 equalizes the amounts of solids entering and leavingreactor 36. This maintains a steady state of reaction in reactor 36. Theoverall solid-to-water ratio in reactor 36 is maintained in the range ofabout 0.2 to 1.2 kilograms solids per liter of water.

Water addition to the reactor is controlled automatically by theabovedescribed feedback loop utilizing level sensor 38, water valve 41,and if desired the described system 75, 76, 77 for recirculating waterwithdrawn by vacuum 74. Similarly, the respective feedback systems formaintaining the reactor temperature and the moisture level of the pasteoperate independently of master controller 101 once the desired setvalues have been provided.

Automated control of components upstream from weigh hopper 22 is alsopossible. Additional lead ingots 12 are added as needed to mill 11.Conveyor 19 may run continuously so long as system 10 is active, e.g.,may be actuated by controller 101 together with valve 63 and controller103. Optionally, conveyor 19 and hopper 22 may be provided with suitablemeans, e.g. a level sensor and controller, for automatically actuatingconveyor 19 in response to a low level of lead oxide in weigh hopper 22.

When system 10 is idle, master controller 101 closes valve 63, causesfilter press controller 106 to deactivate filter press 67, and causesreactor supply controller 103 to deactivate conveyor 33, pumps 47, 48and 53, and feeder 45. Stirrers 44, 46, 54 continue to operate, andwater is added intermittently to reactor 26 as needed to maintain thedesired reactor slurry level. Ball mill 11 can continue to operate sothat lead oxide accumulates in bulk storage tank 18 until needed.

Referring to FIG. 3, an alternative embodiment of the invention utilizesa series of reactors 36A, 36B and 36C for making pastes containingtetrabasic lead sulfate or tri/tetra mixtures. A pipeline 111 providedwith a metering pump 112 feeds partially reacted slurry from reactor 36Ato reactor 36B. A second pipeline 113 provided with a second meteringpump 114 similarly feeds partially reacted slurry from reactor 36B toreactor 36C, wherein the reaction proceeds to the desired degree of leadoxide conversion. Such a cascaded reactor system can provide a greaterthrough speed for the slurry, and can reduce the size and cost of theapparatus, e.g., three 2,000 liter reactors 36A, 36B, 36C could replacea single 20,000 liter reactor 36.

Lead oxide, acid, water, and any desired additives will generally beadded to the first reactor 36A in essentially the same manner as in theembodiment of FIG. 1. Reactors 36B and 36C may have suitable means forproviding additional acid, water, or other ingredients. Reactors 36A-36Cmay have the same volume and can be operated at the same temperature. Inthe alternative, each of reactors 36A, 36B and 36C can be designed andmaintained at a different temperature in order to optimize the reaction,for example, to obtain a higher overall rate of reaction. The equationR=K₁ +K₂ C_(3b) C_(4b), the rate equation for formation of tetrabasiclead sulfate, wherein R is reaction rate, K₁ and K₂ are constants thatvary with temperature, C_(3b) is tribasic concentration and C_(4b) istetrabasic concentration, may be used to determine the most desirablereaction conditions in a multi-reactor system of this type.

FIG. 4A illustrates an apparatus for making pastes containing mixturesof tri- and tetrabasic lead sulfates. Conditions in a first reactor 36Dare adjusted so that the lead sulfate output through a pipeline 121 issubstantially all tetrabasic lead sulfate. Pipeline 121 feeds the slurrycontaining tetrabasic lead sulfate directly into a second reactor 36E.Conditions in reactor 36E are adjusted so that only tribasic leadsulfate forms in reactor 36E. Reactor 36D may, for example, bemaintained at an elevated temperature for forming tetrabasic leadsulfate in the range of 65°-100° C., while reactor 36E is maintained ata lower temperature of 20°-60° C. for forming tribasic lead sulfate. Thefeed rate through pipeline 121 determines the overall composition of thetri/tetra mixture in reactor 36E. The slurry from reactor 36E iswithdrawn through an outlet 122 for dewatering as described above.Reactor 36D is typically larger than reactor 36E, e.g., at least double,especially 2-4 times greater volume if the resulting sulfate mixturecomprises about 40-60% tetrabasic lead sulfate.

The embodiment of FIG. 4A has an advantage over the embodiment of FIG. 1when a tri/tetra mixture of a specific concentration is needed. Althougha single reactor 36 can operate to continuously produce such a mixture,problems arise when delays require that the reactor operateintermittently rather than continuously. When reactor 36 is idle,tribasic lead sulfate will continue to convert to tetrabasic leadsulfate at the elevated temperature of the slurry. This will change theratio of tri- to tetrabasic lead sulfate, so that the desired mixturewill not be obtained when reactor 36 resumes operation. Reactors 36D,36E prevent this problem. Reactor 36E operates at a lower temperaturethan 36D, so that no conversion of tri- to tetrabasic lead sulfateoccurs when the system becomes idle. When reactors 36D, 36E resumeoperation, the resulting paste will have the same tri/tetra ratio.

FIG. 4B illustrates reactors 36D, 36E arranged in parallel rather thanin series. In such an embodiment lines 121, 122 meet at a T-joint 123wherein the slurry mixtures commingle. The combined slurries are thenfed into a mixer 126 to form a homogenous mixture, and then dischargedthrough an outlet line 127 provided with a valve 128 to filter press 67.Such an arrangement could be used, for example, to make a mixture oflead sulfate (PbSO₄) and tetrabasic lead sulfate without any tribasiclead sulfate. Such non-equilibrium mixtures have benefical effects onthe bonding of adjacent paste crystals during curing. Such bonding canincrease plate stength and cycle life.

FIG. 5 illustrates a method of the invention. As described above inconnection with FIG. 1, such a method initially involves a step 201 offorming a lead oxide powder, such as by milling in air. Preformed leadoxide powder may be used in lieu of carrying out step 201. A second step202 then involves forming an aqueous slurry containing the lead oxide,acid and water, and any additives, such as fiber, expander and anyconductive additives. A step 203 of allowing the basic lead oxidereaction to proceed then follows. Generally, this step further involvescontinuously stirring the slurry at a rate effective to keep the solidsin suspension and prevent solid deposits from forming on the reactorwalls and base while maintaining the reaction temperature at the desiredlevel (generally 20° C.-95° C.). Additional water is added as needed tomaintain the desired slurry consistency.

After reaction step 203 is completed, the slurry is withdrawn from thereactor intermittently or continuously at a predetermined rate whichgenerally matches the rate at which the reactants are added to thereactor. The slurry is then partially dewatered (step 204) to form abattery paste of the desired consistency. This step is advantageouslycarried out by first subjecting the slurry to suction through a suitablefilter medium (e.g. belt 73) and then removing further water by pressfiltration, for example, by means of filter press 67. The paste may thenbe fed directly to a pasting apparatus for a further step of pastingelectrode grids (step 205).

In accordance with conventional procedures, the pasted grids (plates)may then subjected to successive steps of flash drying and curing. Flashdrying is needed only if the plates are to be stacked face-to-face forcuring. Flash drying involves heating the plate for a short time so thatthe surface of the plate dries; the interior of the plate remains wet.Flash drying may be carried out by conveying the plates through an ovenheated to at least about 500° F. for a heating time of about at leastabout 8 seconds, particularly 10-15 seconds. Curing generally involvesallowing the plates to stand for a period of at least about 24 hoursunder conditions of elevated temperature and humidity, e.g., 48° C. at95% humidity. Curing may be omitted for positive plates if low free leadoxide is used, or if an oxidizing agent is used to oxidize the freelead. Curing is beneficial but not absolutely necessary in formingnegative plates. If curing is omitted, a method of controlled platedrying is needed.

The battery plates are then assembled into the battery casing, and thebattery is filled with acid electrolyte. The completed battery is thensubjected to conventional formation. During formation a current isapplied to the plate in order to convert lead sulfate, basic leadsulfate and lead oxide to active material (PbO₂ for a positive plate andPb for the negative plate). Ozone treatment or the use of conductiveadditives allows the formation step to be carried out more efficientlyat a high rate for a positive plate, for example, in no more than about8 hours. The thus-formed plates have good reserve capacities.

A variety of optional steps can also be included. For example, the pastemay be reacted with a persulfate prior to dewatering step 204, for thepurposes described above. After dewatering, the paste may be driedand/or milled prior to pasting. A step of recycling water withdrawnduring dewatering step 204 to the slurry of steps 202, 203 is alsouseful for conserving water. Other optional steps may include treatingpasted positive plates with either gaseous ozone or spray-coating withaqueous solutions of persulfate preceding, during or following theflash-drying step. These and the various other operations describedabove in connection with the apparatus according to the invention may beused individually or in combination with steps 202-204.

The process of the present invention has many advantages over knownprocesses for the production of battery paste. The addition of the leadoxide, fiber, expander, and conductivity enhancing agent directly intothe reactor, with the fiber and expander in the form of slurries,eliminates the need for a dry mixing step and provides better dispersionof the solids in the resulting slurry. In the prior batch process, deadspots within the mixture harden into lumps, jamming the paster orcausing the plates to contain irregular chunks. Since the slurry in thereactor according to the invention is continuously stirred and has aconstant consistency due to the intermittent addition of water, no soliddeposits or lumps form in the slurry or on the reactor walls,eliminating pasting problems and greatly reducing cleaning time. This isparticularly useful when switching from a negative to a positive pasterecipe, since additives used in the negative paste are detrimental ifsuch additives contaminate the positive paste.

Further, the specific gravity and addition rate of the sulfuric acid inthe process according to the invention are not as critical as in theprior batch process described above so long as they are known, and"burning" (i.e., oversulfating) the paste with the acid can be avoided.The large excess of water present in the reactor facilitates temperaturecontrol and can virtually eliminate problems with overheating the paste.The water used need not be distilled or preacidified.

The dewatering filtration step of the invention allows the moisture ofthe paste to be controlled by varying the filtration pressure. Thiseliminates the need, for example, to add additional water or acid inorder to obtain a paste of the required consistency or density.Filtration pressure adjustment is also a more effective way to controlthese properties.

The process of the invention can utilize a variety of lead oxidesincluding Barton, leady and nonleady oxides of any crystal structure,and can produce a paste containing either tribasic lead sulfate,tetrabasic lead sulfate, or a mixture thereof at a specified ratio. Theprocess of the invention can also produce other mixtures containing, forexample, monobasic lead sulfate and normal lead sulfate, and leadhydroxide, Pb₃ O₄, or PbO₂. Production of tetrabasic lead sulfate usingthe conventional batch process discussed above generally requiresreacting lead oxide containing very low amounts of free lead withstoichiometric amounts of acid, followed by complete drying of the mix,then forming the paste by addition of water. Tetrabasic lead sulfatesenhance porosity and are preferred for use in making positive platesaccording to the invention. Tribasic lead sulfates are generallypreferred for use in negative plates to obtain batteries with betterlife and cold cranking capacity.

From a production standpoint, the invention allows greater pasteproduction rates, potentially about 4,000 kilograms per hour or more, ascompared to about 1,800 kilograms per hour by conventional batchprocesses. The system can remain idle for an indefinite period and theslurry composition will remain constant. The embodiments of FIGS. 4A, 4Bmake this possible even for pastes containing both tri- and tetrabasiclead sulfates. The paste produced has a more consistent composition, andmay in addition provide a more desirable crystal structure. Thecomposition of the paste may be readily adjusted, eliminating the needto adjust tetrabasic lead sulfate concentration by curing.

According to a further aspect of the invention, a process of making alead acid battery is provided wherein curing of the positive andnegative plates is omitted. Such a process includes an initial step offorming a paste mixture. As described above, this step comprisesreacting a lead oxide having a low free lead content with sulfuric acidin an aqueous slurry, then dewatering the partially reacted slurry toobtain the paste material. If a positive plate is being made thepositive paste may be treated with an agent which forms PbO₂, such asozone or persulfate. Alternatively, PbO₂ or a comparable conductivityenhancing agent can be directly added to the slurry prior to dewatering.As a further alternative, a conventional leady oxide could be used tomake a paste for positive plates, and the plates could then be subjectedto an accelerated curing step in an essentially pure oxygen atmosphereto convert the free lead to lead oxide.

The paste is then applied to a metallic grid to form one or morepositive battery plates. The pasted positive plates are then flashdried, if needed. Negative plates are then prepared as described above.The plates are assembled in a battery casing in a conventional manner,and the battery is then filled with the acid electrolyte and formed(charged).

Such a process can greatly reduce the amount of time needed tomanufacture a lead-acid battery, because a lengthy curing step is notneeded. The main purpose of conventional slow curing of positive platesis two-fold. First, if the paste on the plate contains free lead, curingoxidizes it. Second, curing generates the desired tribasic/tetrabasiccrystal structure for optimizing the strength and electrical performanceof the plate. The amount of tetrabasic lead sulfate is determined duringpaste formation, and no subsequent adjustment is needed. Free lead canbe removed or oxidized. Elimination of the curing step in this mannercan allow a complete lead-acid battery to be manufactured in less than24 hours.

It will be understood that the foregoing description is of preferredembodiments of the invention, and that the invention is not limited tothe specific forms described. For example, the apparatus according tothe invention may advantageously be employed to produce plates forbatteries other than lead-acid batteries, and paste materials other thanbattery pastes. As feeder 6, other types of feeders such as volumetric,screw or vibratory feeders may be employed. Dewatering apparatus 67 mayinclude other types of filtration and separation devices, such ascentrifugal dewatering devices. These and other modifications of theinvention may be made without departing from the scope of the inventionas expressed in the appended claims.

We claim:
 1. An apparatus for producing a battery paste material,comprising:a tank having a discharge outlet; a feeder disposed to feed aparticulate solid into the tank; an inlet disposed to introduce waterinto the tank to form a mixture with the particulate solid; a conveyorhaving a porous conveyor belt disposed to receive the mixture from thedischarge outlet of the tank on a surface of the porous conveyor belt; afiltration device capable of drawing a portion of the water from themixture through the porous conveyor belt to form a paste material; and apaster disposed to receive paste from the filtration device and applythe paste to a series of grids.
 2. The apparatus of claim 1, furthercomprising:a stirrer disposed in the tank; a heat control system capableof maintaining the temperature of the tank at a predetermined level; anda valve disposed in the discharge outlet, which valve prevents dischargeof the mixture to the filtration device when closed, and permits suchdischarge when open.
 3. The apparatus of claim 1, wherein the dischargeoutlet is centrally positioned on a side wall on the tank, and the tankfurther has a drain valve outlet disposed proximate the bottom of thetank for removal of material that accumulates at the tank bottom.
 4. Theapparatus of claim 1, further comprising a recycle line connecting thefiltration device to the tank, and a pump disposed to recycle waterdrawn from the mixture by the filtration device through the recycle lineto the tank.
 5. The apparatus of claim 1, further comprising a line forconducting the mixture from the discharge outlet to the filtrationdevice, the line including a discharge outlet valve for regulating flowof the mixture from the tank, and a heat exchanger that adjusts thetemperature of the mixture before the mixture is fed to the filtrationdevice.
 6. The apparatus of claim 1, further comprising a line enteringthe tank near its bottom for introducing a gas into the tank.
 7. Theapparatus of claim 1, wherein the filtration device further comprises avacuum filtration device.
 8. The apparatus of claim 1, wherein thefiltration device further comprises a filter press.
 9. The apparatus ofclaim 8, further comprising:a sensor disposed to measure the watercontent of the paste emerging from the press filtration device; and apressure control system for adjusting pressure exerted by the pressfiltration device to increase the pressure if the water content of thepaste determined by the sensor is above a predetermined value, ordecrease the pressure if the water content of the paste determined bythe sensor is below a predetermined value.
 10. The apparatus of claim 8,wherein the filter press includes a second conveyor belt disposed abovethe porous conveyor belt, whereby the mixture on the porous conveyorbelt is press filtered between the conveyor belts as it moves along theconveyor belts.
 11. The apparatus of claim 10, wherein the dischargeoutlet of the tank is disposed to deposit the mixture on an uppersurface of the porous conveyor belt, and the filtration device furtherincludes a vacuum filtration device disposed beneath the porous conveyorbelt ahead of the press filtration device.
 12. The apparatus of claim10, further comprising:a sensor disposed to measure the water content ofthe paste emerging from the press filtration device; and a pressurecontrol system for adjusting pressure exerted by the press filtrationdevice to increase the pressure if the water content of the pastedetermined by the sensor is above a predetermined value, or decrease thepressure if the water content of the paste determined by the sensor isbelow a predetermined value, including a pair of rollers disposed inopposed positions above the second conveyor belt and below the porousbelt to form a nip through which the mixture passes, and a controllerconnected to receive a signal from the sensor indicating the measuredmoisture level, which controller varies the pressure exerted by therollers on the conveyor belts in response thereto.